In the packaging industry, it is common to transport cases on conveyors formed from bar chain. Typically, a pair of continuous loops, each with its own drive, are provided to transport such cases to work stations at which they are filled, emptied, closed, opened, or otherwise treated. Typically, the nature of the load being transported dictates that each loop include two or more drive motors, be they electric, pneumatic, or hydraulic. The provision of a plurality of drive motors in each loop serves to evenly distribute the load between the various drives of the system. However, if the motors are not totally synchronous, one motor will eventually assume the entire load of the system, while the others will perform little or no work at all. Such a situation not only is inefficient, but generally gives rise to failures and the need for service and repair, such being inconsistent with the purpose for providing plural drives in the system.
It has been found that when two or more motors are used in a drive system, a degree of slack in the chain is necessary at each motor to assure that the load is equally distributed among them. When slack is absent at a first motor, the remaining motor or motors are required to perform the work of the first motor, the absence of slack indicating that there is no disparity in force within the chain on opposing sides of the motor.
In the prior art, it has been taught to provide motors in such conveyor systems with torque limiting or slip clutches. In such systems, if one drive motor begins to outrun the other, the clutch slips until the load distribution changes. However, in such prior art systems, the correction is inefficiently achieved by inhibiting the operation of the motor doing all of the work, instead of increasing the operational speed of the motor doing virtually no work at all.
Further, the prior art teachings in this regard suggest that complex systems are necessary to attain motor synchronization, such systems being given to wear, adjustment problems, and repeated service interruptions.